The present invention relates to a method for manufacturing semiconductor devices each element-separated by a so-called field oxide film.
There has been known an example wherein a so-called SOQ (Silicon On Quartz) substrate in which a support substrate is of an insulative quartz support substrate, is used to obtain a semiconductor device having excellent characteristics such as high frequency characteristics and radiation-proof characteristics.
In a thermal oxidation step for forming a field oxide film using a LOCOS (Local Oxidation of Silicon) method, a mask pattern obtained by patterning a silicon nitride film is used in such an SOQ substrate.
The formation of the silicon nitride film is generally performed by an LP-CVD (Low Pressure Chemical Vapor Deposition) method performed at low pressure.
There has been known, for example, a configuration wherein in a semiconductor device manufacturing method in which element or device isolation is done by means of a field oxide film, a mask pattern applied to a LOCOS method, i.e., a thermal oxidation step is formed of a silicon nitride film formed by a CVD method with a view of preventing undesired thinning of an element isolation area or region due to an etching step (refer to a patent document 1 (Japanese Patent Publication Laid Open Number 2005-353745)).
It has been known that the mask pattern obtained by patterning the silicon nitride film formed by the LP-CVD method in particular, which is used in such a thermal oxidation step, produces extremely large tensile stress of 1 GPa (Giga pascal) to 2 GPa over a full range from room temperature (low temperature.) to about 800° C. (high temperature) at which the thermal oxidation step is done.
Problems involved in the related art will now be explained with reference to FIG. 5.
FIG. 5 is a photographic diagram for explaining the related art. The drawing shows a stacked or laminated structure in which a base or substrate silicon oxide film 134 and a silicon nitride film 140′ are laminated on an SOQ substrate 120 in this order.
Described concretely, an SOQ substrate is prepared in which a silicon layer 130 having a thickness of 100 nm is laminated on a quartz support plate 120 having a thickness of 625 μm.
Next, a laminated structure is obtained in which a substrate silicon oxide film 134 having a thickness of 160 nm is formed, and a silicon nitride film 140′ having a thickness of 300 nm is formed on the substrate silicon oxide film 134 by an LP-CVD method.
The so-obtained laminated structure is observed by an SEM (Scanning Electron Microscope).
As shown in FIG. 5, a crack 150 has occurred which extends through (cut longitudinally) the silicon nitride film 140′, the substrate silicon oxide film 134 and the silicon layer 130 and reaches within the thickness of the quartz support substrate 120.
Thus, when the SOQ substrate is used and the silicon nitride film, i.e., the mask pattern for the thermal oxidation step is formed on the SOQ substrate, the quartz support substrate and the thin structure provided on the quartz support substrate may cause cracks due to the tensile stress and the difference between the silicon nitride film and the quartz support substrate in the extent of thermal expansion. As a result, there is at risk of impairing electrical characteristics of a manufactured semiconductor device.
The inventors of the present invention have found out that in forging ahead with extensive investigations, a mask pattern used in a thermal oxidation step in a LOCOS method is formed by a plasma CVD (PE-CVD: Plasma Enhanced Chemical Vapor Deposition) method and the form of the mask pattern is devised thereby to enable the solution of the conventional problems, thus leading to completion of this invention.